Alt Text for Images in PDFs

In this tutorial, you will learn about Alt Text for Images in PDFs. We cover key concepts, practical examples, and best practices to help you master this topic.

Alt text for images in PDFs means providing a textual description for each figure, photo, chart, or graphic in the tag tree so that screen reader users understand the content and purpose of every non-text element.

What You'll Learn

You will learn how alt text is stored in tagged PDFs, how to add and edit alt text using Acrobat Pro, how to write concise and useful descriptions, how to handle complex images like charts, and how to verify alt text with a screen reader.

Why It Matters

Without alt text, a screen reader announces "Figure" or reads the filename or says nothing at all. The user misses all information conveyed by the image. For complex graphics like financial charts, this is equivalent to removing the entire data section.

Real-World Use

A pharmaceutical company publishes clinical trial results as PDFs. Each chart showing efficacy data has alt text describing the trend, key data points, and conclusions. Blind medical researchers can understand the chart without seeing it, enabling them to evaluate the trial results.

Alt Text in the Tag Tree

flowchart TD
  A["Figure Tag"] --> B["Alt Text Property"]
  B --> C["Short description of image"]
  A --> D["ActualText (optional)"]
  D --> E["Full text replacement"]
  A --> F["Nested content (if any)"]

Adding Alt Text in Acrobat Pro

1. Open the Tags panel.
2. Find the <Figure> tag for the image.
3. Right-click the tag and choose Properties.
4. In the Tag tab, enter the alt text in the "Alternate Text" field.
5. Set the Type to "Figure" if not already set.
6. Click Close and verify the alt text appears in the Tags panel.

// Acrobat JavaScript to add alt text to all figures
var tags = this.getTags();
for (var i = 0; i < tags.length; i++) {
    var t = tags[i];
    if (t.structType === "Figure" && (!t.alt || t.alt === "")) {
        t.alt = prompt("Enter alt text for this figure:", t.alt || "");
        console.println("Set alt for Figure " + i);
    }
}

Writing Good Alt Text

# Quality check for alt text in PDF
import pdfplumber

def check_alt_text(pdf_path):
    """Check if all images in the PDF have non-empty alt text."""
    with pdfplumber.open(pdf_path) as pdf:
        for page_num, page in enumerate(pdf.pages, 1):
            images = page.images
            for img in images:
                alt = img.get("alt", "")
                if not alt or alt.strip() == "":
                    print(f"Page {page_num}: Image at ({img['x0']:.0f},{img['y0']:.0f}) has no alt text")
                elif len(alt) > 200:
                    print(f"Page {page_num}: Alt text is very long ({len(alt)} chars)")

check_alt_text("report.pdf")
# Example output: "Page 2: Image at (100,500) has no alt text"

Handling Complex Images

For charts and diagrams, provide both a concise alt text and consider using an ActualText entry (a longer description that replaces the entire content). Some PDF producers also add a visible data table next to charts.

Common Mistakes

1. Alt text like "image" or "photo" — This provides no useful information. Describe what the image shows, not what it is.

2. Forgetting decorative images — Decorative images that convey no information must be marked as artifacts, not figures. Otherwise, screen readers announce "Figure" with no content.

3. Alt text from file name — "chart-2026-Q3.png" is the filename, not a description. Do not leave alt text populated by the filename.

4. Too much detail for simple images — A logo does not need "Blue square with white text reading DodaTech in bold sans-serif font." Keep it concise.

5. Missing alt text for linked images — If an image is also a link, the alt text should describe the link destination, not just the image content.

Practice and Challenge

1. What is the difference between alt text and ActualText in a PDF?
2. How do you mark an image as decorative (artifact) in Acrobat?
3. Why should chart alt text include data trends and key numbers?
4. How long should alt text typically be?
5. How do you verify alt text using NVDA or JAWS?

Challenge: Take a PDF containing at least 5 images including a chart, a logo, a photo, and two decorative elements. Add appropriate alt text to each non-decorative image and mark decorative images as artifacts. Verify the alt text with a screen reader and write a brief report of your decisions.

FAQ

Mini Project

Select a 3-page PDF with at least 5 images (a photograph, a pie chart, a logo, a decorative border, and a screenshot). Add appropriate alt text to each non-decorative image. Mark decorative elements as artifacts. Export the tag tree as a text report and verify that alt text is present for all figures.

What's Next

Continue to Accessible Tables in PDFs to learn how to make tables navigable with screen readers, then proceed to Form Fields in PDFs.